The present invention relates to a light-selective prism for use in a projector or the like.
In projectors, light emitted from an illuminating optical system is modulated by means of a liquid crystal light valve in response to image information (image signal), and the modulated light is projected onto a screen.
In projector for projecting color images, a colored light separating optical system is provided for separating light emitted from an illuminating optical system into three colored lights, and a colored light combining optical system for combining three modulated lights emitted from three liquid crystal light valves. As a colored light combining optical system, a light-selective prism (cross dichroic prism) is used for example, having two types of selective films formed at an almost xe2x80x9cXxe2x80x9d shape interface of four rectangular prisms.
Selective prisms are conventionally manufactured by independently preparing four rectangular prisms and then sticking them together. A method for manufacturing a selective prism of this kind is described, for example, in JAPANESE PATENT LAID-OPEN GAZETTE No. H11-352440 disclosed by the present applicant.
However, where four rectangular prisms are prepared independently in the above manner, sometimes the desired optical characteristics of the light-selective prism are not obtained. One cause of this is that there is variation in the refractive index of each rectangular prism. This variation of refractive index can occur, for example, by using light-transmissive parts of different lots to form each rectangular prism.
The object of the present invention is thus to solve the drawbacks of the prior art discussed above and to provide a technique for improving optical characteristics of a light-selective prism.
At least part of the above and the other related issues are solved through the method for manufacturing a light-selective prism in the present invention. The light-selective prism has a substantially regular tetragonal columnar outer shape and includes two kinds of selective films, formed on an almost X shape interface, each selective film selecting colored light having wavelengths of a predetermined range. The manufacturing method comprises the steps of: (a) preparing a block formed of a light transmissive member; (b) cutting the block along at least one plane perpendicular to a first direction so as to obtain a plurality of first small blocks whose dimension in the first direction is substantially equal to a predetermined dimension; (c) forming a first selective film on a cut face of at least one of the plurality of the first small blocks; (d) sticking the plurality of the first small blocks together so as to obtain a first processed block in which the first selective film is situated at an interface of adjacent two first small blocks; (e) cutting the first processed block along at least one plane perpendicular to a second direction that is substantially perpendicular to the first direction so as to obtain a plurality of second small blocks whose dimension in the second direction is substantially equal to the predetermined dimension; (f) forming a second selective film on a cut face of at least one of the plurality of the second small blocks; (g) sticking the plurality of the second small blocks together so as to obtain a second processed block in which the second selective film is situated at an interface of adjacent two second small blocks; and (h) obtaining at least one light-selective prism from the second processed block.
With the method of the present invention, the light-selective prism is manufactured from the single block. Thus, variation in refractive index of light-transmissive parts constituting the light-selective prism that occurs due to differences among lots can be reduced, and as a result of this, it is possible to improve optical characteristics of the light-selective prism.
In the above method, it is preferable that the step (b) includes polishing the cut faces of the first small blocks; and that the step (e) includes polishing the cut faces of the second small blocks.
By so doing, the film formation face of a first small block on which a first selective film is formed, and the film formation face of a second small block on which a second selective film is formed, can be planarized, so the degree of adhesion of the first and second selective films to the first and second small blocks can be improved, respectively.
Here, the step (h) may include cutting the second processed block so as to obtain a plurality of the light-selective prisms.
By so doing, the plurality of the light-selective prisms can be obtained from the single block.
In the above method, the step (d) may include sticking the plurality of the first small blocks together such that the outer shape of the block is reproduced; and the step (g) may include sticking the plurality of the second small blocks together such that the outer shape of the block is reproduced.
Here, it is preferable that the step (d) includes sticking the plurality of the first small blocks together such that each portion of the light transmissive member constituting each first small block is placed in the same location within the block; and the step (g) includes sticking the plurality of the second small blocks together such that each portion of the light transmissive member constituting each second small block is placed in the same location within the block.
Even within a single block, there are instances in which there is variation in the refractive index depending on spatial position. Thus, by proceeding in the manner described above, spatial variation in refractive index of light-transmissive parts constituting a light-selective prism can be reduced, and it becomes possible to improve the optical characteristics of the light-selective prism.
Alternatively, the step (d) may include sticking the plurality of the first small blocks together in a state such that adjacent two first small blocks are dislocated in a direction substantially perpendicular to the first and the second directions; and the step (g) may include sticking the plurality of the second small blocks together such that an outer shape of the first processed block is reproduced.
In this arrangement, the plurality of second small blocks can be stuck together by utilizing the dislocation formed between two adjacent first small blocks. Therefore, when sticking the plurality of second small blocks together, the first selective film portions divided due to cutting out of the plurality of second small blocks can be easily arranged within the same plane.
Here, it is preferable that the step (d) includes sticking the plurality of the first small blocks together such that each portion of the light transmissive member constituting each first small block is placed in substantially the same location within the block; and the step (g) includes sticking the plurality of the second small blocks together such that each portion of the light transmissive member constituting each second small block is placed in the same location within the first processed block.
In the above method, it is preferable that the first selective film is a blue light reflecting film for selectively reflecting blue light; and that the second selective film is a red light reflecting film for selectively reflecting red light.
Within the light-selective prism, the first selective film is formed in a divided state, but the second selective film is formed in a continuous state. The sensitivity of the human eye is higher to red light than to blue light. Therefore, by setting the first selective film and the second selective film to a blue light reflecting film and a red light reflecting film respectively, segmentation of the first selective film does not stand out compared to the case of the reverse setting.
In the above method, it is preferable that the block has a substantially rectangular parallelopiped shape, and that the at least one plane perpendicular to the first direction and the at least one plane perpendicular to the second direction are set to planes inclined by about 45 degrees with respect to each side of one pair of opposing faces of the block.
By so doing, the light-transmissive member that forms a block can be utilized without waste, and at least one light-selective prism can be obtained.
In the above method, it is preferable that the light transmissive member is a member having a thermal conductivity of at least about 5.0 W/(mxc2x7K).
By so doing, the temperature rise of the light-selective prism per se can be reduced. Further, when an optical component of relatively large heat generation such as a polarizing plate or retardation plate is attached to the light-selective prism, temperature rise of these optical components can be reduced as well.
In the above method, it is preferable that the light transmissive member is a uniaxial crystal member, and that the first and second directions are set to directions substantially perpendicular to an optic axis of the uniaxial crystal.
Here, the uniaxial crystal member may be a monocrystalline sapphire member or a rock crystal member.
Uniaxial monocrystalline members can be used as light-transmissive members with relatively high thermal conductivity. However, when linear polarized light enters a uniaxial monocrystalline, the state of polarization thereof is changed in some cases. If the relationship of the first and second directions and the optic axis of the monocrystal is set as mentioned above, the state of polarization of linear polarized light will not be changed.